This invention relates to mounting methods and apparatus for piezo-electric discs such as force transducers and radio reference crystals.
Force transducers in which a piezo-electric disc is mounted on its edge between two opposing anvils are known. U.S. Pat. No. 3,274,828 issued to Pulvari, U.S. Pat. No. 3,386,292 issued to R. A. Watson et al., and U.S. Pat. Nos. 3,891,870 and 4,020,448 issued to J. P. Corbett all show such transducers.
In Corbett U.S. Pat. No. 3,891,870, for example, a plate-like piezo-electric crystal disc is mounted on its edge between a brass-clad stainless steel lower platform and a brass-clad stainless steel upper force platform. As described by Corbett, the edge of the crystal wears a permanent indent in the brass shims on the stainless steel platforms, thereby seating the crystal. The platforms are biased together so that the crystal remains in the seats. A load or force applied to the crystal through the force platform causes a proportional change in the resonant frequency of the crystal. Accordingly, the load or force applied to the crystal can be measured by determining the new resonant frequency or the change in resonant frequency of the crystal. Corbett shows and describes circuits for monitoring the resonant frequency of the crystal. Other circuits for performing this function are shown and described in the other patents mentioned above.
The piezo-electric discs in transducers of the kind described above may be either crystals such as quartz, or ceramics such as barium titanate, lithium tantalate, or lithium niobate. The frequency response of the disc is generally extremely sensitive to the angular orientation of the disc relative to the axis of applied force. For precise, repeatable, linear frequency response it is desirable to apply the force to the smallest possible areas on the edge of the disc. In addition, to provide a transducer with greater sensitivity and/or greater usable load span, it is desirable to provide a disc mounting which will permit increased maximum loading of the disc in the limited areas of contact between the mounting and the edge of the disc. On the other hand, these piezoelectric materials are extremely brittle. Although they have relatively high compressive strength, their tensile strength is relatively low, and they are easily chipped, cracked, or fractured during mounting and subsequent loading.
A radio reference crystal disc, which is also piezo-electric, may be mounted similarly to the piezo-electric discs in force transducers of the type described above. A mounting of this type is particularly preferred as a shock mounting for enabling the crystal to withstand forces due to high accelerations and the like. The crystal is typically mounted along its axis of zero force-frequency coefficient, and the crystal is heavily loaded to maintain it in its mounting despite acceleration forces or the like. Many of the considerations which apply to mounting piezo-electric discs in force transducers also apply to such mountings of radio reference crystals.
In view of the foregoing, it is an object of this invention to provide improved methods and apparatus for mounting piezo-electric discs in applications of the type described above.
It is a more particular object of this invention to provide methods and apparatus for mounting piezo-electric discs which permit increased loading of the disc while limiting the area of contact between the edge of the disc and the mounting.
It is another more particular object of this invention to provide methods and apparatus for mounting piezo-electric discs which increase the load capacity of the disc and, in the case of force transducers, improve the linearity and repeatability of the transducer.